It's not really for the cruise lines to say whether cruising is safe, even for those who are vulnerable. It is up to the individual to decide whether the potential risks outweigh the benefits of a cruise holiday. Cruise lines have put in place protocols to minimise the risks, but there can be no guarantees. P&O sem to be responding correctly to any significant covid outbreak onboard, but they are between a rock and a hard place when it comes to applying tighter restrictions, some accept them as necessary, others hate them, so it's a fine line for the cruise lines who are struggling to survive with their reduced earnings and high debts.